The kidney contains several distinct epithelia that, in their aggregate function, are responsible for formation of the urine and the maintenance of fluid and electrolyte homeostasis. We are studying the roles of these epithelia in the regulation of the excretion of water, urea, sodium, and chloride. The two central approaches are 1)to microdissect the epithelia from the kidney and to study their functions in vitro, and 2) to make antibodies to each of the cloned transporters expressed along the renal tubule to allow direct study of the regulation of the transporters. The studies measure the transport by the epithelia, the level of expression of various transport proteins in renal tubules, intracellular signaling in individual renal tubule segments, and the levels of nucleic acids (mRNA) for proteins that are involved in the regulation of both the transport and metabolic functions of the epithelia. The current focus is on: 1) mechanisms of NaCl retention in hypertension and in extracellular fluid volume expanded states (congestive heart failure, cirrhosis, nephrotic syndrome); 2) the molecular physiology of the urinary concentrating mechanism; 3)development and characterization of polyclonal antibodies against renal transporter and channel proteins; 4) molecular physiology of renal NaCl transport regulation including regulation of the epithelial sodium channel (ENaC) by vasopressin and aldosterone.